shower rebuild

I am currently rebuilding from a disasterous shower failure that rotted the entire bathroom subfloor. It was a fully tiled shower stall with durock backer and PVC floor membrane. It was done by a contractor and I found a razorblade cut in the membrane probably made while cutting the drain and few other violations of the proceedures I've been reading on various info sites. The original shower had failed in less than 10 years and the rebuilt one destroyed the bathroom floor in five years. The shower walls are still good with tile over durock and I would like to keep them if possible and replace only the floor. I cut out the damaged PVC membrane which extended behind the backer. I can't get a new one behind there without tearing out the walls. I don't want another tile floor anyway for this basic reason: You are depending on a moisture barrier which you cannot see hidden below the tile. If it fails, you don't know until your subfloor starts to cave in. Now I want to install a moulded plastic shower base so that the moisture barrier is the surface I'm standing on so if it fails I will see it. My problem is joining the top flange of the shower base with the tile walls in a way that won't leak. I have read how this is done if you are building the walls from scratch over the base, but I want to keep my walls. The base is not a perfect fit. This is more of a problem than I thought it would be. I'm now considering custom building a base in-place with glass/epoxy (I'm a boatbuilder). I'm starting to think it might be easier to tear down the walls and start from scratch. I would appreciate any advise.

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